1. Field of the Invention
The present invention relates to electrostatically servo-controlled pendular micro-sensors including a pendular element arranged to be subject to a force representative of a physical quantity to be measured, at least one pair of electrodes including a mobile electrode integral with the pendular element, a fixed electrode facing the mobile electrode, and a system for detecting the displacements of the pendular element controlling an electrostatic servo-control system. The servo-control system applies voltages to the pair of electrodes for applying return electrostatic forces to the pendular element.
The present invention more particularly relates to a servo-control system for the pendular element and its operation mode.
2. Discussion of the Related Art
The above-mentioned micro-sensors include a cell for detecting a physical quantity. The cell includes a slidable or rotatable part, referred to as an "pendular element", elements for suspending the pendular element, and elements for applying onto the pendular element a force representative of the physical quantity to be measured.
The detection cell is achieved through micro-machining or micro-etching of materials such as silicon or quartz.
Such pendular micro-sensors can be used for detection of acceleration, inclination, pressure and, more generally, any physical quantity from which a force acting on the pendular element can be generated.
By way of example, the sensitivity of a pendular micro-accelerometer is due to the inertia phenomenon, the force applied to the pendular element being the product of the pendular element's mass by the acceleration value. For a pressure micro-sensor, there is provided, for example, a diaphragm separating the medium in which the pressure is to be measured from a medium in which the reference pressure is contained, the pendular element contacting the diaphragm or being integral therewith in order to respond to a force resulting from the difference between the pressure to be measured and the reference pressure.
The electrostatically servo-controlled pendular micro-sensors further include a system for detecting the displacements of the pendular element, acting on an electrostatical servo-control system that generates on the pendular element a return force opposing the force generated by physical quantity. Since the pendular element is maintained in quiescent position, the intensity of the physical quantity is determined by measuring the reaction force.
Various systems for detecting displacements of the pendular element are known, especially so-called "capacitive" detection systems, in which the position of the pendular element is determined from the capacitance variation of at least one capacitor including a fixed electrode and a mobile electrode integral with the pendular element.
Generally, in order to provide a return electrostatic force on the pendular element, one applies, between a fixed electrode of the sensor and an electrode integral with the pendular element, a d.c. voltage ranging from a few volts to several hundred volts, depending on the applications. An electrostatic attraction, having an intensity proportional to the squared applied voltage, is generated between the electrodes and tends to bring the pendular element closer to the fixed electrode of the sensor.
When the direction of the return electrostatic force is to be monitored, at least two electrode pairs, disposed on both sides of the pendular element, are used. Then, the pendular element can be restored by applying a determined voltage to the appropriate electrode pair. Another alternative is to constantly maintain two opposite electrostatic forces while acting on the unbalance of their intensities so as to generate a resulting return force having the same direction as the direction of the highest electrostatic force.
Two techniques are mainly used to vary the intensity of the electrostatic force. The first technique consists in varying the value of the d.c. voltage applied to the electrodes. The second so-called "pulse width modulation" (PWM) technique consists in using a chopped d.c. voltage and varying the duty cycle so as to modulate the mean value of the servo-control voltage.
In order to simplify the detection cell, the servo-control electrodes are frequently used for detecting the position. Then, steps are to be taken so that the detection and control voltages do not interfere.
For a better understanding, exemplary detection cells, sensitive to a physical quantity, are described hereinafter.
FIGS. 1-3, which are referred to by way of example only, schematically show various types of sensitive cells including two pairs of return electrodes.
FIGS. 1 and 2 schematically show sensitive cells of inertial micro-sensors capable of detecting acceleration, inclination, angular frequency, etc. FIG. 3 schematically shows the sensitive cell of a pressure micro-sensor.
Each sensitive cell 1 comprises a pendular element 2 including, on two opposite sides, electrodes 3a, 4a respectively facing two fixed electrodes 3b, 4b formed on the body 5 of the sensitive cell 1. The electrodes 3a, 3b, 4a, 4b form two pairs of electrostatic return electrodes 3, 4, acting in complementary directions.
Suspension means 6 maintain the pendular element 2 in the absence of return electrostatic forces. In FIGS. 2 and 3, the suspension means 6 correspond to the base 6 of the pendular element 2 and forms therewith a vibrating beam.
The sensitive cell 1 of FIG. 3 further includes means for applying on the pendular element 2 a force representative of the pressure to be measured. Here, this means is a diaphragm 7 separating the external medium of the sensitive cell 1 from an internal medium 8 at reference pressure P. The pendular element 2 includes a protruding portion 9 contacting diaphragm 7.
The equivalent electric diagram of the sensitive cells of FIGS. 1-3 is shown in FIG. 4a and includes the two above-described electrode pairs 3 and 4.
In FIGS. 1-3 the sizes of the various elements and the intervals separating electrodes 3a from 3b, or 4a from 4b, are not drawn to scale but are arbitrarily drawn in order to facilitate legibility of the drawings. In practice, a cell 1 has a thickness of a few tenths mm and a length of a few mm; the distance between the electrodes being only a few tenths .mu.m.
In FIGS. 1-3 are not shown the electric connections for accessing electrodes 3a, 3b, 4a, 4b. Various technological methods are available and are not part of the invention.
However, it should be noted that the pendular element 2 can be made of a conductive material, such as silicon, and electrodes 3a and 4b can be made of doped silicon. Such a method uses a single electric connection for electrodes 3a and 4a, as indicated in FIG. 4b.
The above-described micro-sensors exhibit numerous advantages due to their small size and their good sensitivity. However, it is noted that sometimes the information provided by such micro-sensors drifts in time, for a given physical value.